Hendrix Systems Lab — Rebuilding a Guitar Hero’s Signal Chain (2025)

This repository contains the Python, ngspice, and Makefile sources used to generate all figures and data for the article
“Hendrix, Systems Engineer: Rebuilding a Guitar Hero’s Signal Chain with SPICE”
(submitted to IEEE Spectrum, 2025).

It reconstructs the analog signal path used by Jimi Hendrix (1966–1970) as a cascade of nonlinear stages—Fuzz Face, Octavia, Wah, Uni-Vibe, and Marshall stack— and models them as analog systems using ngspice and Python tools for visualization and audio demonstration.

All SVG figures appearing in the manuscript and supplementary ODT files were generated directly from these scripts with no manual edits.

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🔧 Environment

Tool	Version tested
Python	3.11
ngspice	42.1
NumPy	≥ 1.26
SciPy	≥ 1.13
Matplotlib	≥ 3.8
soundfile	≥ 0.12

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📁 Contents

File	Purpose
Makefile	Automates ngspice runs and SVG generation. Main entry point.
fuzzface_ge_pnp_posgnd.cir, fuzzface_si.cir	Germanium and silicon Fuzz Face transistor models.
octavia_behavioral.cir, octavia_transformer_rect.cir	Two Octavia variants (behavioral and transformer/rectifier).
vox_wah_param.cir	VOX V847 wah-wah parameterized deck.
univibe_frozen_lfo.cir, univibe_frozen_lfo_sweep.cir	Uni-Vibe phase-shift network (fixed and swept LFO).
bode_quotient.py, merge_bode.py, plot_spice.py, wr_collapse.py, wah_q_table.py, temp_bias_plot.py	Python utilities for parsing WRDATA logs and producing Bode, phase, and Q-factor plots.
chain_hendrix.py, hendrix_lab.py	End-to-end simulation of the Hendrix signal chain in Python; generates .wav stems and spectrograms.
echo_ir.py	Simple delay/feedback impulse response plotter.
plot_octavia_nuclear.py, plot_octavia_pretty.py	Idealized and SPICE-driven Octavia rectifier plots.
requirements.txt	Python dependencies (create if not present).
README.md	This file.

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▶️ Running the Build

To reproduce all ngspice analyses and plots:

make all

This will:

1. Run each .cir netlist through ngspice in batch mode,
2. Generate .dat WRDATA files in out/data/,
3. Invoke the Python utilities to collapse, merge, and plot,
4. Produce IEEE-sized SVGs in out/figs/.

For a clean rebuild:

make clean

Generate audio demonstrations

python hendrix_lab.py

Exports .wav stems for each block in the signal chain (fuzz, Octavia, wah, Uni-Vibe, Leslie, amp) at 48 kHz / 24-bit.

Individual figures

Examples:

python plot_spice.py --ieee --in out/data/fuzz_si_ac.dat
python merge_bode.py out/data/wah_*.dat --out out/figs/wah_overlay.svg
python plot_octavia_nuclear.py
python plot_octavia_pretty.py

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🧩 Data Flow

.cir  →  ngspice WRDATA (.dat)
.dat  →  wr_collapse.py / bode_quotient.py
.csv  →  merge_bode.py / wah_q_table.py
.svg  →  figures in manuscript

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📊 Figure Mapping

Figure (ODT)	Script(s)	Source data
Fig. 1–6 Fuzz Face	bode_quotient.py, plot_spice.py	fuzzface_si.cir, fuzzface_ge_pnp_posgnd.cir
Fig. 7 Octavia	plot_octavia_nuclear.py, plot_octavia_pretty.py	octavia_transformer_rect.cir
Fig. 8–9 Wah	merge_bode.py, wah_q_table.py	vox_wah_param.cir
Fig. 10–11 Uni-Vibe	merge_bode.py	univibe_frozen_lfo*.cir
Spectrograms	hendrix_lab.py	internal synthetic chain

All plots were exported as SVG at IEEE column width (3.25 in).

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🧠 Notes

• The Fuzz Face decks use corrected emitter connections (Q1 C1 B1 E1 …, etc.).

• Germanium vs. silicon variants differ only in transistor models and polarity.

• The Makefile sets ngspice options for reproducible WRDATA output:

  .options numdgt=15 reltol=1e-5 abstol=1e-9 vabstol=1e-6
  .control
  set wr_singlescale
  set wr_vecnames
  set wr_noindex
  .endc
  

• The wah_q_table.py utility scans Bode data to find –3 dB edges and computes Q-factors automatically.

• The chain_hendrix.py and hendrix_lab.py scripts normalize only once at the end of the chain for realistic gain staging.

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🧾 License and Citation

All code and SPICE decks © 2025 [Rohan Puranik]. Released under the MIT License unless stated otherwise.

If you use this material in research or teaching, please cite:

[Rohan Puranik], “Hendrix, Systems Engineer: Rebuilding a Guitar Hero’s Signal Chain with SPICE,” GitHub repository, 2025. DOI: (Zenodo DOI if minted)

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🧩 Acknowledgments

Based on circuit analyses of the Fuzz Face, Octavia, Vox V847 Wah, and Uni-Vibe effects described in the Hendrix FactPack 2025 and accompanying article. ngspice 42.1 and Python 3.11 were used for all analyses and figure generation.